Recovery of viscous petroleum materials



De@ 7, 1965 P. J. cLosMANN ETAL 3,221,313

RECOVERY OF VISCOUS PETROLEUM MATERIALS Filed Aug. 12. 1963 CHARLES S. MATTHEWS BY mw THEIR ATTORNEY -This invention relates to the recovery of petroleum materials from subterraneanvformations containing viscous.

tar-like petroleum materials. More particularly, the invention is directed to thermally driving petroleum materials from such formations in which the formation structure is incompetent. As used herein in reference to petroleum-bearing formations, the term incompetent refers to formations that may be competent at the reservoir temperatures, but tend to slump or cave into any voids created therein when they are heated to a temperature at which the petroleum in 4the formations -is mobile;

Throughout the world there are variousknown `locations wherein the earth contains large deposits of tar sands.

For example, one of the most extensive and best known deposits of this type occurs in the Athabasca district of Alberta, Canada. In the tar sands in such deposits, the oil typically has a density approaching `or even greater than that of water. The Athabasca tar sands extend for many miles and occur in varying thicknesses of up to more than 200 feet. Although in some places 'the Athabasca tar sands are disposed practically on the. surfacefof the earth,. generally they are located under an overburden which ranges in thickness from a few feet to'as much as 100 ormore feet in depth. The tar sands located at these depths constitute one of the worlds largest presently known petroleum deposits. In these sands, the oil content ranges between about and 20% by weight, al-

-though sands with lesser or greater amounts of oil content are not unusual. Additionally, the sands generally lcontain small'arnouuts of water in the range of from about 1 tov 5%'b'y weight.

The oil present in and recoverable'I from Athabasca tar sands is usually a rather viscous material rangingv in specitic gravity from slightly below 1.00 to about 1.04 or somewhat greater. At atypical reservoir'temperature, e.g., about 48 F., this oil'is immobile, having a viscosity exceeding several thousand centipoises. At higher temperatures, such as temperatures aboveabout 200". F. this oil becomes mobile, with viscosities of less than about 343 centipoises, and the tar sands are incompetent. Since this tarry material does not generally comand a'very high price, particularly when in its crude state, its separation and recovery must involve a minimum of expenditure in order to economically attractive for commercialv prac tice.

One method of recovering oil from tar sands is to strip mine the sands with relatively conventional mining apparatus and to'process the Amined sand to separate 'oil therefrom. This method has the obvious disadvantage that it is impractical for usein recovering oil from tar sand deposits located at considerable depths -below the surface of the earth. Therefore, the use of strip mining to recover oils from tar sands, such as are located in the Athabasca district of Alberta, Canada, is restricted to those deposits located near the surface of the earth.- As a result, only a relatively small percent of the total tar deposits in the Athabasca district can be so recovered.

Another method of recovering oil from formations containing viscous petroleum materials is through the use of thermal-drive techniques which thermally lower the viscosity of oil within the formations and drive the oil so lowered in viscosity to production wells where they may Patented Dec. 7, '1965 be produced to the surface of the earth through conventional production techniques. Typically, such thermaldrive techniques employ an injection well and a production well extending into the reservoir formation. In operation, a hot uid, usually steam because of its economic advantages, is introduced into the formation through thev injection well. Upon entering the formation, the heat. transferred by the hot fluid functions to lower the viscosity of oil therein while the tlow of the hot uid functionsv to drive thevoil to the production well where it may be produced to the surface of the earth. lt has been found that conventional thermal-drive prooesses do not generally prove effective in recovering oils from tar sands. The reason for this ineffectiveness resides primarily in the fact that the tar sands, at the natural temperatures of the deposits, are not sufficiently permeable to allow the steam or other hot fluids to pass through i the deposits to effectively lower the viscosity of oil therein. In order to overcome this ditculty resulting from the impermeable nature of tar sands, it has been proposed to extend a fracture between the injection and production wells prior to or during the injection. Such a fracture is expected to facilitate the ow of hot uid'through the fracture' bounded bjf'the'l tar sand deposit and, thus to provide a'means for heating lthe oil within the deposit.

The use of fracturing to facilitate thermal-drive processes has not, however, proved suliicient, in' itself, to make thermal-drives in tarl sand deposits practical. Such fractures tend to close as soon as the pressure utilized to create them is.relieved. Upon this occurrence, the unheated tar sand reverts to its impermeable state and isnot subject to' production with conventional thermal-drive processes. In a competent formation the closing ofsuch a fracture can be avoided by introducing propping agents, such as granular materials, into the fracture to hold it open. This method, however, is inelective in respect to an incompetent oil-bearing formation such as a tar sand. As soon as the walls of the fracture become heated, the incompetent formation slumps between the grains of the propping agent and the permeabilityis lost.

In the case where atar sand deposit has been fractured to communicate injection and production wells, a thermaldrive uid can be injected into the deposit by injecting itvthrough a fracture that is maintained full of fluid at a. pressure suicient to hydraulically support the weight of the materia-ls above the fracture. Such a pressure, which generally corresponds to the overburden pressure, is hereinafter referred to as the fracture propping pressure. This method of hydraulically maintaining the fracture in an open state is practical so long as the fracture propping pressure required -to maintain the fracture open plus the additional pressure required to drive fluids through the fracture, the total Ibeing the pumping pressure, does not exceed the fracturing pressure ofthe formation. It should be noted that the fracturing pressure, or formation parting pressure, is the pressure required for a fluid to break down the structure of the' 'formation by creating a fracture. The fracturing pressure isf generally considerably higher than the fracture propping pressure. The latter isrprimarily affected by the bulk density of the overburden and usually' corresponds to about 0.7 pound per square inch per foot of overburden. The fracturing pressure exceeds the fracture propping pressure by amounts primarily alected by the strength of the bonds between the particles of the formation. In tar sands at reservoir temperatures this difference may be in the order of 300 p.s.i.g. in excess of the fracture propping pressure. Once the pumping pressure applied to the thermal-drive fluid approaches the fracturing pressure of the deposit, this method of hydraulically propping the fracture proves to be very undesirable since an uncontrolled fracturing of the deposit is apt to create a hazard to the operating personnel and a azzurra V 3 loss of the thermal-drive fluid. lt is noted that although the initial pressure required to maintain a fracture in an open state, i.e. the fracture propping pressure, may be we'll below the fracturing pressure of the deposit, the pumping pressure required to maintain a ow through the fracture may increase to the fracturing pressure after a period of injection of the thermal-drive fluid. This results from the fact that asthe fracture walls are heated the fracture accumulates oil which has been warmed, but is still viscous, and more pressure is required to force uids to ow through the fracture. If the pumping pressure is raised to the fracturing pressure of the formation, uncontrolled fracturing arid its undesirable effects are ylikely to occur.

It is, therefore, an object of the present invention to provide a process for recovering petroleum materials from vtarsand stratum. It is another object of the invention to from incompetent tar sand deposits through relatively inexpensive thermal-drive means.

Basically, the present inventionprovides a process for driving petroleum materials from a subterraneanpetrokum-bearing formation to a producing well penetrating the formation from which the materials may be produced.

-To commence the process, an injection well is completed into the formation at a spaced location with respect to the production well penetrating thereinto. After the injection vwell is so formed, a fracture is extended through the formation into communication with the production and injectionvwells anda hot gas, having a temperature at which the petroleum is mobile is pumped into said fracture' at a pressure that is-less than the fracturing pressure of theforrnation, but is suthcient to force fluid to ow through the fracture to the production well.

The term hot'gas is used herein to refer to a gaseous or gas-containing fluid that is substantially chemically inert to petroleum, but has a temperature at which the petroleum in the formation being treated is mobile. The particularly' preferred hot gases comprise steam, such as saturated, low quality (Le. water-containing) or super- 'heated steam in which the gas phase predominates at the .pressures existing within the formation being treated. Qther suitable hot gases include mixtures of steam with air, combustion products and/or other gases, and in general, substantially any heated and chemically inert gas ormixture of gas and liquid at a temperature that mo- -bilizes thepetroleum and maintains the gas phase at the pressures at which the hot gas is used. The hot gases may containcomponents that enhance their tendency to entrain petroleum materials, such as ammonia or amines, etc., in their gas phase and/or ammonium or alkali metal bases in their liquid phases.

When the pressure required to force fluid to flow throughthe fracture closely approaches the fracturing pressure of the formation, the pumping of the hot gas into the fracture is terminated. At this point, a vapor-free liquid capable of entraining viscous petroleum materials, preferably by dissolving and/or emulsifying them, is pumped into the fracture through the injection well to remove viscous petroleum materials that have accumulated in the fracture. After sucient petroleum materials have been removed from the fracture to reduce the pressure requiredfor said pumping to a pressure closely approaching the fracture propping pressure, the pumping of vaporfree liquid is terminated and the pumping of the hot gas is resumed. It should be noted that during the pumping i of the vapor-free petroleum entraining liquid it is preferable, but not essential, that the pumping pressure be kept below the fracturing pressures, to whatever extent is possible in maintaining ow into the output well. Since this is an incompressible fluid, the application of higher pressures may create new or extended fractures but, as long as the ow patterns extend into the output well, the new or extended fractures will be disposed in locations in which they are useful to the production process.

In a. particularly preferred embodiment of the present process, an incompetent petroleum-bearing formation is selectively heated so that it is heated most rapidly in the portions immediately adjacent to the fracture. This selective heating is accomplished by cooling the inner sides of the outer walls of the wells that communicate wit-h the fracture into ywhich the hot gas is injected, particularly in the regions adjacent to the petroleum-bearing foi-.mation above the depth of the fracture. This causes thc-'rate of heat transfer to be the most rapid in the immediate vicinity of the fracture and causes t-he petroleum in the walls of the yfracture to be the rst to become mobile. Two important advantages are obtained by such a selective heating. First, it minimizes the hazard of blowouts to the surface near the injection well. Wherever an incompetent formation is heated it tends to yield to allow luids to flow in response to pressure. A hot gas being injected Iinto a fracture within an earth formation exerts the same pressure on all portions of the formation that are contacted by the -hot gas. If any portions of the formation yield, the hot gas expands to continue to exert pressure on these portions. The portions of the formation above the fracture and adjacent to the injection well are positioned so that, if the heating is nonselective, they are the portions that are rst'heated by the first arriving and hottest portions of the hot gas. -fField experience has demonstrated that a nonselective heating of an incompetent petroleum bearing formation by injecting a hot gas at a pressure exoeeding the propping pressure of a fracture, can and does result in hazardous blowouts near the injection well. Second, the selective heating enhances the advantages that are provided .by alternately pumping through the fracture a hot gas and a liquid capable of dissolving or emulsifying viscous petroleum materials. It controls the rates at which heat is transferred so that the first heated portions of the formation are the walls of the fracture. The

4 walls of the fracture are swept by the fluids that are pumped through the formation. Thus, the portions of petroleum that are irst mobilized by the heat transferred to the formation a-re the portions disposed for ready solution and/ or emulsication by the vapor-free petroleum entraining liquid alternated with the hot gas. ln the initial stages of an application of the invention, this provides a significant improvement in respect to the over-all eticiency obtained where the heating is non-selective and heat is transferred to portions of the petroleum disposed in locations that are not swept by the vapor-free petroleum entraining liquid.

In practicing the selective heating procedure it is particularly important that the lheat radiating from the hot gas be controlled, by radiation, conduction and convection, to'cool the inner side of the outer wal1 of the injection well in a region bounded by the top of the incompetent petroleum-bearing formation and the fracture through which the hot gas is pumped. Although the rate at which the hot gas is pumped through the fracture. can 'be controlled so that the temperature of the gas that reaches the output well is less than the vtemperature at which the petroleum is mobilized, it is advantageous to similarly cool the similarly located portions of the output wells, and is generally advantageous to cool the walls of all portions of all wells used inthe process. This cooling should maintain the Walls of the wells at temperatures below those of the hot gases and preferably at temperatures b :elow those at which the petroleum is mobile. Methods rectangle;

- ihack to the pumping of hot gas are repeated until the pressure required to force fluid to ow through the fracture during the pumping of hot gas remains substantially constant and below the fracture propping pressure of the formation. communication between the injection and production wells is formed through the -formation and this facilitates the continuous injection of an economically adavntageous hot gas such as steam through the formation at pressures below the fracture propping pressure of the formation. After a zone of the formation has been swept clear of immobile petroleum, the pressure required to pump steam through the formation will be less than the fracture propping pressure of the format-ion, since it will no longer be necessary t support the overburden by hydraulic pressure. When :this stage is reached, a steam'drive process can be continued very economically, since the hazards and expenses of the high pressure injection procedures are avoided.

',The enumerated and other objects of the invention and In this manner, an expanding zone of fluid thedetails of the inventive process will lbecome apparent when viewed in light of the following detailed description and accompanying illustrations, wherein:

FIGURE l diagrammatically illustrates a vertical section of a ta-r sand formation having the process of the present invention applied thereto; and, l FIGURE 2 illustrates a section 'of the yfractu-re illustrated Iin FIGURE l taken on line 2 2 thereof after the process of the invention hasbeen in effect for a periodof time.

Referring now `vto FIGURE 1, therein is illustrated an lincompetent tar sand formation 10 having relatively impermeable formations 11 and 12 located at the upper and lowei sides thereof, respectively. The formation 10 has 'extending thereinto injection and production walls 13 and 14, respectively, to accommodate the process of the present invention. Although the illustration in FIGURE 1 and the subsequent description referring to this illus-v tration is only directed to use of the process with a single injection well and a single production well, it is to be understood that the number of wells and the well patterns may vary wit-hout departing from the linvent-ion. |For example, tihe wells :may be arranged in a conventional tive-spot pattern with four wells forming the corners of a rectangle and a fifth well`formed in the center of this W'ith such an arrangement, the corner wells could be used as injection wells while the center well is used as a production well. Conversely, with, the same arrangement the center well could be used as -an injection well while the corner wellsare used as production wells.

In the FIGURE l illustration, -the injection well 13 is shown as lbeing provided with a casing string 15 having an injection string 16 extending concentrically therethrough. The production well 14 is similarly constructed with a casing string 17 having a production string 20 excould be insulated through means of any desired 'insula-y tion and these strings could be provided withperforated lower ends rather than open lower ends.l

In addition to the insulating means, the wells may be provided with a system to cool the formation adjacent .to the well bores. For example, as illustrated in the well 13, a packer 21 is set in the casing string of the well above the open lower end thereof and a coolant conduit 22 is run through the casing string to a position slightly above To complete the system, an exhaust open-A the packer. ing 23 is formed in the upper end of the cap onv the cas.- ing string 15. Through this arrangement, a coolant may be introduced into the well through the conduit 2,2, circulated through the annulus around the insulation pack and injection string, and exhausted through the opening 23. This coolant functions to cool the area of the tar sand formation around the lower end of the casing string and thus avoids blowouts in this area which might result from excessive lowering of viscosity of petroleum inaterials in this area due to heat imparted thereto from the well. Naturally, alternative cooling arrangements to that illustrated in the well 13 couldvbe used without departing from the process of the invention.. Y

In application of the process of the invention, once production and injection wells are completed into the formation desired to be treated, as illustrated in FIGURE l, a fracture as indicated by the numeral 24-is hydraulically induced and extended between these wells. The fracture can be formed at any selected depth. preferablyv below the center of the tar sand, by any of the means for selecting the depth of a fracture known to those skilled 'Y into the fracture through the injection well to heat the,

formation adjacent the fracture and drive petroleum materials from the tar sand formation to the production well; Upon initial injection, the hot gas is pumped through the fracture at a pumping pressure significantly greater than the fracture propping pressure, but less than the fracturing pressure of the tar sand formation 10. The fracture propping pressure of the fracture 24 may be conveniently estimated in p.s.i. as being equal to 9.7 times the depth of the fracture from the surface of the earth expressed in feet. The fracturing press-ure and the fracture propping pressure of a given petroleum hearing formation may bedetermined experimentally, by measuring the pressure required to initiate the fracture and the minimum pressure at. which a liquid can be pumped through the fracture. Where the hot gas is steam, its initial injection at pressures above the fracture propping pressure, but less than the fracturing pressure, has the advantage that in addition to preventing the incompetent formation 10 from caving in and closing the fracture, it increases the rate of heating by introducing the steam at the highest feasible temperature; but, the pumping pressure should be kept close to the fracture propping pressure where the formation isv particularly incompetent and/ or heterogeneous.

Referring now to FIGURE 2, therein is illustrated an enlarged vertical section of a portion of the fracture 24 after a hot gas such as steam has been injected therethrough for a period of time. The upper and lower sections of FIGURE 2 show the tar sands of the formation 10 in their original state. The central section of FIG- URE 2 illustrates the process which occurs as hot gas passes through the fracture 24. In the central section it can be seen that molten tar and sand grains, as designated by the numerals 25 and 26, respectively, are falling due to gravity segregation, from the upper section of the tar sand formation through the fracture to the lower section of the tar sand formation, upon which they settle n a redistributed fashion. As so redistributed and freed of immobile tar, the sand grains form a relatively permeable mass relative to the impermeable mass formedby the tar sand in its initial state. It is also noted that as the molten tar 25 and sand grains 26 fall through the fracture, the upper wall of the fracture, formed by the upper' section of tar sand, migrates upwardly due to the removal of material therefrom. The dotted line duplications of tar droplet 25 and its fragments 25a as well as the generally horizontal arrows, indicate the fluid flow and the path of the tar droplets, many -of which are entrained and swept out of the fracture by the flow of hot gas.

After the hot gas has been injected into the fracture 24 for a period of time, molten tar that is still viscous accumulates to an extent where the pumping pressure required to maintain the ow of hot gas and petroleum materials (c g., tar) through the fracture must be `increased to a point approaching the fracturing pressure of the tar sand formation 10. At this point, the injection of hot gas is terminated, since injection at the fracturing pressure of the tar sand formation would, result in uncontrolled fracturing of the formation and a hazardous loss of the -hot gas. It is noted that the pressure of hot gas within the fracture' 24 can be readily monitored by indicating means communicating with the injection string 16 used to convey the gas into the fracture.

Uponthe termination of the hot gas injection, the injection well 13 is utilized to inject a vapor-free petroleum entraining liquid through the fracture 24 to remove the buildup of hydrocarbon materials therefrom. The purpose of this injection is to clean the fracture to a point where its permeability will support an economical hot gas drive such as a steam drive at pumping pressures under the fracture propping pressure of the formation 10. At this point, itis noted that the purpose of alternately injecting the hot gas and the entraining liquid into the fracture 24 is to raise the temperature ofthe formation and drive petroleum materials that accumulate in the fracture to the well 14 where theymay be produced in a conventional manner through the production string 20.

The vapor-free petroleum entraining liquid injected through the well 13 may take the form of a petroleum emulsifier or a petroleum solvent. Preferably, the liquid is introduced at a temperature which is high relative to the temperature of the materials being treated in the fracture, in order to accelerate its rate of reaction and preserve the temperature attained in and around the fracture. Entraining liquids capable f emulsifying petroleum may take various forms, such as aqueous caustic (e.g., NaOH or KOH) or aqueous sodium, potassium, ammonium, carbonates, borates or the like salts of strong bases and weak acids, which may contain surface active materials as dissolved or dispersed components of the liquids. Particularly efficient tar entraining liquids are described in Can. Patent 639,050 and U.S Patent 2,882,973. Liquids that are petroleum solvents may also take various forms, such as kerosene, isopropyl alcohol, diesel fuel, or any other suitable liquid petroleum solvents that may be readily available. All of these liquids, either the emulsiliers or the petroleum solvents, have the advantage that they function to drive or sweep petroleum materials from the fracture while substantially the only fluids in the fracture arc substantially incompressible liquids. The exact concentrations of the components of the multicomponent liquids used for this purpose are generaly determined experimentally in order to obtain optimum results in the economics of the sweeping out of the fracture. The chemical composition of these liquids is not particularly critical, since those which are less efficient in the entraining of petroleum can be pumped for longer periods. The most important properties of these liquids are their incompressibility at the temperature and pressure existing in the fracture and their capability of entraining at least some petroleum.

After the pressure required to force the vapor-free liquid solvent or emulsifier through the fracture has decreased to one that approaches the fracture propping pressure and is substantially less than the fracturing pressure, the vaporfree liquid injection is terminated and steam or other hot gas is injected into the fracture in a manner corresponding substantially to the initial injection of hot gas that followed the formation of the fracture. This hot gas injection is continued as was thc first hot gas injection until the pumping pressure required to maintain the flow once again exceeds the fracture propping pressure by a significant amount, or approaches the fracturing pressure of the formation 10. At this point vapor-free petroleum entraining liquid injection is repeated in a manner corresponding to that of the firstvapor-free liquid injection.

The alternating steps of injecting hot gas and injecting a vapor-free petroleum entraining liquid are thus repeated in the above manner until the pressure required to force fluid through the fracture during the injection of the hot gas remains substantially constant and below the fracture propping pressure of the formation. At this point, the step of injecting a liquid emulsifier or solvent is no longer repeated and recovery is accomplished only by the continued injection of'an economical hot gas such as steam into the formation 10 through the injection well 13. 1t is noted, that the thermal-recovery process at this time corresponds very closely to conventional steam drive processes wherein steam is continuously injected into a producing formation through an injection Well extending thereinto and produced from a production well extending thereinto at a location spaced from the injection well.

In the present process, a continuous forward steam drive is facilitated when the formation 10 has been heated to an extent sufficient to substantially reduce the viscosity of hydrocarbons therein and, at the same time, the zone of permeable formation adjacent to the fracture issufiiciently large and permeable to permit the petroleum materials -so reduced in viscosity to be driven therethrough at pressures below the fracture propping pressure of the formation l0. The latter conditions, namely that where the zone of permeable formation adjacent to the fracture is sufficiently large and permeable, occurs largely as a result of the heating and redistribution of sand grains, the removal of tar` and the migration of the fracture boundaries. as was described and illustrated with respect to FIGURE 2. Specifically, upon redistribution and heating, the sand grains form a relatively permeable mass in the fracture 24. This mass is particuarly effective in facilitating the ow of material through the formation 10, since it accumulates to a thickness in excess of that of the original fracture 24 due to the upward migration of the upper surface of the fracture. This permeable mass, together with th'e reduced viscosity of the petroleum material entering thereinto. due to the heat absorbed by the for-mation, facilitates the driving of petroleum material through the formation to the production well 14 through direct steam drive from the injection well 13.

Once the bottom of the fracture becomes a substantially continuous and significantly thick layer of redistributed and cleaned sand grains having a temperature sufcient to mobilize petroleum materials entering the fracture, it is possible to effect a steam drive through the fracture at a pressure less than the over-burden pressure, with the over-burden supported on a mass of sand that is too thick to be closed by any slumping between its grains. This sitnation is particularly desirable, since it lowers the cost of steam injection into the formation and also alleviates the necessity of maintaining high pressure steam equipment on hand at the injection site. It can thus be seen that the present invention may be applied to a relatively large field Example 1.-Delermining pressures to be employed A pair of injection and production wells were completed into an incompetent Athabasca tar sand. The wells were equipped as illustrated ,in FIGURE 1 to inject fluids through a fracture at a depth of 200 feet. The wells were fractured by injecting water at a temperature of about 75 F. into the tar sand which had a' temperature of about 48 F. The water injection required a surface pressure of about 410 p.s.i., indicating the fracturing pressure to .be about 496 p.s.i.g. at the depth of the fracture. In alternately injecting a hot gas and a vapor free petroleum entraining liquid, steam injection was initiated at 150 p.s.i.g. and the pumping pressure was increased as necessary to maintain ow until the pumping pressure reached about 300 p.s.i.g. Each time pressures of about 300 p.s.i.g. were required, the steam injectionwas replaced by the injection of heated aqueous 0.1 to 0.2 percent by weight sodium hydroxide solution, and the latter was pumped through the fracture until liow was obtained at pumping pressures corresponding to about 200 vp.s.i.g.

' 'at the depth of the fracture, at which times the steam injection was resumed.

A significant amount of tar was thus produced by injecting a hot gas at pumping pressures maintained between a fracture propping pressure and a formation fracturing pressure, using the pressures measured during the fracturing of the wells, and alternating the hot gas in' jection with a vapor free petroleum entraining liquid injection when the plugging of the fracture required the use of pumping pressures significantly close to the fracturing pressure.

l Example [1 Producing lar and expanding the zone of production A series of wells was completed in the form of a fivespot pattern with a central injection well and four producing wells opened into an incompetent Athabasca .tar sand. The. producing wells were spaced about 90 feet from the injection well and a temperature observation well was spaced about 70 feet from the injection well. The injection and production wells were fractured, as described in Example I, except for using the dilute sodium hydroxide solution as the fracturing fluid. The methods and apparatus described in aforementioned patent U.S. 3,142,336, were used to cool the interior of the injection well. v

After two months of alternately injecting the steam and the dilute sodium hydroxide as described in Example I, the average rate of production of tar mounted to about 20 barrels per day per well. The temperature measurements in the observation well indicated that the vertical heat flow was increasing due to the upward migration of the fracture and those measurements, correlated with the pumping pressures required to maintain flow between the wells, indicated the development of a swept zone through which steam could be continuously injected at pumping pressures lower than the fracture propping pressure.

Example IlI.--Uncooled steam injector In a production well that was fractured at a depth of 200 feet within an incompetent Athabasca tar sand, steam was injected through a production tubing string in a well that was not provided with means for cooling the interior of the well to a temperature below the temperature at which the tar is mobile. The steam wasinjected at a pressure of 170 p.s.i.g. andsteam ow was maintained for about one-half hour; at which time the steam erupted to the surface.

v It should be noted that this steam injection pressure was greater than the fracture propping pressure, about I44 p.s.i.g. lat the depth of the fracture, but-was materially less than the formation fracturing pressure,y which was about 400 p.s.i.g. at the depth of the fraction.

The foregoing description and illustrative examples of the inventiveprocess are merely intended to be explana tory thereof. Various changes in the details of the described process may be made, within the scope of the appended claims, without departing from the spirit of the invention. For example, any well arrangement may be used wherein a fracture may be extended to facilitate the process of the invention. Furthermore, it is anticipated that the invention could be applied to viscous petroleum formations other than tar sand formation so long as the structure of the formation corresponds substantially to that of the described tar sand formation in its behavior during application of the process.

We claimv as our invention:

1. An improved process for recovering petroleum tars from shallow incompetent tar-sand reservoirs where the danger of a blowout is imminent when pressurized, heated fluids are injected into the reservoir to displace the viscous petroleum tars comprising:

(a) penetrating such a shallow, incompetent tar-sand reservoir with at least one production well' and at least one injection well spaced from one another;

(b) establishing fluid communication through said reservoir between saidinjection well and said production well;

(c) injecting a heated, gaseous huid via said injection well through said established fluid communication between said injection well and said production well at a pressure suicient to maintain said fluid communication -but below the fracturing pressure of said reservoir;

(d) terminating the injection of said heated, gaseous fluid when the pressure required for injecting itappreaches said fracturing pressure of said formation;

(e) subsequently injecting a substantially non-compressible, petroleum tar entraining liquid via said injection well and owing it through said established fluidv communication between said injection well and said j production well at a pressure greater than the fracture propping pressure of saidformation to-remove petroleum tar, ingressing into said established lluid communication; and

(f) thereafter continuing said injecting of said heated, gaseous uid below said fracturing pressure of said tar-sand reservoir.

2. A method according to claim 1 wherein the noncompressible, hydrocarbon entraining liquid is at a temperature higher than the natural formation temperature. 3. The process according to claim 1 wherein the steps of terminating the injection of the heated, gaseous fluid` and of injection of a substantially non-compressible, hydrocarbon entraining liquid is repeated if the pressures from the subsequent injection of said heated, gaseous lluid approaches that of the fracturing pressure of the formation.

4. A process according to claim 1 wherein the injection well is maintained at the natural reservoir temperature to prevent the substantial heating of said reservoir contiguous to said well except where the heated uids are injected.

5. A process according to claim 1 wherein the heated, gaseous lluid is steam.

6. A process according to claim 1 wherein the iluid communication is established by fracturing.

(References on following page) 11 References Cited by the Examiner 3,042,114

UNITED STATES PATENTS 11/1957 Marx et n1 16s-4o x 31165208 12/1958 Dixon 166--40 5 3,167,120 1/1961 Huntington 16s- 40x 3,167,121 10/1961 Crawford 166-11 12/1961 Ten Brink 16s-11 x 4/1962 Willman et a1 16s- 4o x Willman 166-40 X Closmann 166-40 X Doscher 166-11 Jorda 166-11 X Pryor 166-11 X Sharp 166-11 CHARLES E. OCONNELL, Primary Examiner. BENJAMIN HERSH, Examiner. 

1. AN IMPROVED PROCESS FOR RECOVERING PETROLEUM TARS FROM SHALLOWS INCOMPETENT TAR-SAND RESERVOIRS WHERE THE DANGER OF A BLOWOUT IS IMMINENT WHEN PRESSURIZED, HEATED FLUIDS ARE INJECTED INTO THE RESERVOIR TO DISPLACE THE VISCOUS PETROLEUM TARS COMPRISING: (A) PENETRATING SUCH A SHALLOW, INCOMPETENT TAR-SAND RESERVOIR WITH AT LEAST ONE PRODUCTION WELL AND AT LEAST ONE INJECTION WELL SPACED FRO FONE ANTHER; (B) ESTABLISHING FLUID COMMUNICATION THROUGH SAID RESERVOIR BETWEEN SAID INJECTION WELL AND SAID PRODUCTION WELL; (C) INJECTING A HEATED, GASEOUS FLUID VIA SAID INJECTION WELL THROUGH SAID ESTABLISHED FLUID COMMUNICATION BETWEEN SAID INJECTION WELL AND SAID PRODUCTION WELL AT A PRESSURE SUFFICIENT TO MAINTAIN SAID FLUID COMMUNICATION BUT BELOW THE FRACTURING PRESSURE OF SAID RESERVOIR; (D) TERMINATING THE INJECTION OF SAID HEATED, GASEOUS FLUID WHEN THE PRESSURE REQUIRED FOR INJECTING IT APPROCHES SAID FRACTURING PRESSURE OF SAID FORMATION; (E) SUBSEQUENTLY INJECTING A SUBSTANTIALLY NON-COMPRESSIBLE, PETROLEUM TAR ENTRAINING LIQUID VIA SAID INJECTION WELL AND FLOWING IT THROUGH SAID ESTABLISHED FLUID COMMUNICATION BETWEEN SAID INJECTION WELL AND SAID PRODUCTION WELL AT A PRESSURE GREATER THAN THE FRACTURE PROPPING PRESSURE OF SAID FORMATION TO REMOVE PETROLEUM TAR, INGRESSING INTO SAID ESTABLISHED FLUID COMMUNICATION; AND (F) THEREAFTER CONTINUING SAID INJECTING OF SAID HEATED, GASEOUS FLUID BELOW SAID FRACTURING PRESSURE OF SAID TAR-AND RESEVOIR. 